Orwell Astronomical Society (Ipswich)
Construction Of An Equatorial Mount
There are many types of mountings for the telescope maker to choose from, each with its good and bad points. I'm not going to discuss the merits of each type here, as that would take too long, but simply will describe an equatorial mount that I constructed in the late 1980s. My main interest is astrophotography so I wanted a mount that could be easily and accurately driven for long exposures. I also required a design that was relatively simple and straightforward to construct as I didn't have access to workshop equipment. The completed mount is shown in the photo below and, although it may look complicated, the only tools necessary to build it were a file, a drill and a hacksaw (plus a little mathematics!)
Numbers in the photograph are related to paragraphs in the description below as [1], [2], etc.
I designed the mounting from the bearings outwards. I bought four 50 mm inside diameter (ID) bearings in Norwich. Unfortunately, the company that supplied them has now ceased trading but it should be possible to find an alternative local supplier. Having acquired the bearings I next bought some 50 mm outside diameter steel bar from Macready's of Londona for the Right Ascension (RA) and declination (dec) axes [1]. This was as far as I took the project for some time until I found some 114 mm ID steel tube in a scrapyard. The tube was just big enough to accommodate the RA bearings with a little room to spare, and I was able to secure them in position using three equally spaced shims. I manufactured the shims oversize and filed them down progressively until I was able to achieve a really tight fit, sufficient to hold the bearings securely in the tube.
Having found a method for fixing the bearings I needed a way to join the tubes. Cutting the tubes to give a flat end perpendicular to the tube axis is easy. Simply wrap a piece of paper with a straight edge around the tube and carefully align the overlapping ends so that the straight edges meet. Cutting along the edge of the paper gives a straight end to the tube. Cutting one tube to fit another at 90° [2] is a little more complicated. After a little thought and a lot of scribbling I derived the following formula:
y = rb - (rb2 - ra2sin(θ2))0.5
as the general expression for joining at right angles two tubes of radii rb and ra where rb ≥ ra. The term θ is the angle around the axis of the tube with radius ra. When rb = ra, as is the case here, the equation simplifies to:
y = r cos(θ).
Plotting values of y on a paper strip, wrapping it around the tube end and cutting along the plotted line gave a shape that joined the tubes together. (Note that this approach ignored the thickness of the tube so a little filing was needed for a perfect fit!)
Joining the polar axis tube to the 12 mm steel baseplate required a straight cut at an angle Ψ across the tube [3]. The deviation, x, from a straight tube end in this case is:
x = r sin(θ) / tan(Ψ).
I used the following paper strip (reduced scale below) based on the above formula to make the cut.
Once all the pieces were cut I welded them together. This can be difficult - you may have to find someone to do it for you. The remaining problem is joining the 50 mm steel shafts into their respective housings [4] and [5]. Again the ideal way to do this would be to weld them but this requires great accuracy so I decided instead to cast them into place with concrete. I filed a flat onto the end (side) of each shaft to prevent rotation. While the concrete set, the shafts were held in place by their respective tubes with bearings in place, great care being taken in positioning. The final touch in assembling the mount was to put spacer rings on the axes before sliding them onto their bearings; this ensured that all the thrust was put onto the bearings giving a smooth motion.
This completed the components of the mount so next I turned to the concrete pier [6]. For this I constructed a wooden box to hold four 20x300 mm pieces of threaded rod to which the mount would be bolted [7]. I dug deep foundations to give a solid, wide base from which the pier rises. I aligned the box for casting the pier north-south as follows. First I drove a stick vertically into the ground, then, on a fine day, I observed the shadow cast by the Sun near midday (GMT). As the Sun crosses the meridian the shadow of the stick points due north. Note however that the Sun does not generally cross the meridian at 12.00 exactly; it can be up to 16 minutes fast or 14 minutes slow due to the equation of time, so I made an allowance for this. (This topic is covered in most good general astronomy books.) Having found the north-south line I aligned the edge of the square box and began to pour concrete. Once the concrete had set I approximately positioned the polar axis [8] on the pier. I aligned the polar axis to the true pole by inserting a purpose-made telescope into the bearings and sighting on Polaris. I then offset the mount by the correct amount in the right direction to the true pole. Note that the holes cut in the baseplate were elongated to allow a slight rotation about the vertical in the east-west direction: this allowed for small errors in the initial alignment of the concrete pier.
Clearly the mount will not be balanced if a telescope is fixed to it, and a counterweight is required. The mass of the counterweight can be determined by attaching an empty container half way along the dec axis and gradually adding sand until the telescope is balanced: the weight of sand can then be measured. I made a counterweight from concrete, which has a density of approximately 2500 kg/m3. Knowing the weight required I was able to calculate the volume of concrete needed. I constructed a mould from a ring of 250 mm plastic tubing with a 6.4 mm steel ring as the core; this could easily be adjusted along the 50 mm axis. I fixed the counterweight on the axis using two retaining rings constructed as pieces of steel tube with holes drilled and tapped to take a couple of bolts. I slid the counterweight up and down until I found the balance point and then locked it in place with the retaining rings.
I purchased drives, comprising worm gears and synchronous motors, for both axes, from Dark Star Telescopesb. The declination drive [10] is not used at present but "locks" the dec axis when observing. The RA drive [11] is supplied from a 12V battery via the simple circuit shown below which gives approx 240V output at 50 ±10 Hz. A hand held unit contains the 50k potentiometer which controls the frequency output and hence the speed of the motor while guiding.
[a]
[b]
Mike Harlow